(a) Field of the Invention
The present invention relates to a hydraulic control system for a continuously variable transmission. More particularly, the present invention relates to a hydraulic control system for a continuously variable transmission in which only a single accumulator is used, thereby minimizing the number of elements in the hydraulic control system.
(b) Description of the Related Art
The transmission functions to deliver engine drive power to the drive wheels. There are three basic types of transmissions: (a) the manual transmission, in which the driver manipulates a shift lever to control shifting into different speeds and ranges; (b) the automatic transmission, in which shifting into the different forward speeds is automatically controlled according to various driving conditions; and (c) the continuously variable transmission, in which shifting is automatically performed over a single, large range and on a continuous basis when in either forward or reverse mode.
Among the different types of transmissions described above, the continuously variable transmission is ideal for many situations as it offers the convenience and ease of control of the automatic transmission, while providing many additional advantages such as reduced fuel consumption, enhanced power transmission performance, and reduced weight. In the continuously variable transmission, one pulley is mounted on an input shaft and another pulley is mounted on an output shaft, and a diameter of the pulleys is varied to realize shifting. Such a continuously variable transmission is referred to as a belt-type continuously variable transmission.
A hydraulic control system for controlling the belt-type continuously variable transmission will now be described with reference to FIG. 2.
Hydraulic flow is generated by an oil pump 102 to create hydraulic pressure in the hydraulic control system. This hydraulic pressure passes through a primary regulator valve 104 such that the hydraulic pressure undergoes initial control to a predetermined level of line pressure. The line pressure is then supplied to a secondary regulator valve 106 and a secondary pulley 108. The line pressure undergoes secondary control to a predetermined level in the secondary regulator valve 106, after which the line pressure is fed to a solenoid supply valve 110. The solenoid supply valve 110 distributes the hydraulic pressure to first, second, third and fourth solenoid valves S1, S2, S3 and S4. The line pressure supplied from the secondary regulator valve 106 is also supplied to a shift ratio control valve 111, a pressure regulator valve 114, and a torque converter feed valve 122.
The hydraulic pressure supplied to the shift ratio control valve 111 by the secondary regulator valve 106 is fed to a primary pulley 112 according to control by the second solenoid valve S2, thereby effecting changes in a diameter of the primary pulley 112. Such variations in the diameter of the primary pulley 112 result in gearless shifting.
Further, the hydraulic pressure supplied to the pressure regulator valve 114 by the secondary regulator valve 106 is then supplied to a manual valve 116 according to control by the third solenoid valve S3. The hydraulic pressure is subsequently supplied to a forward pressure line 118 or a reverse pressure line 120 depending on how the driver positions a select lever. If the hydraulic pressure is supplied to the forward pressure line 118, a first friction element C receives the hydraulic pressure, and if the hydraulic pressure is fed to the reverse pressure line 120, a second friction element B receives the hydraulic pressure.
The hydraulic pressure supplied to the torque converter feed valve 122 by the secondary regulator valve 106 is stabilized in the torque converter feed valve 122, then supplied to a lock-up clutch control valve 124. The lock-up clutch control valve 124 subsequently supplies the hydraulic pressure to the torque converter and elements requiring lubrication according to control by the fourth solenoid valve S4.
An accumulator 126 is provided on each of the forward pressure line 118 and the reverse pressure line 120. The accumulators 126 and 128 stabilize the operation of the first and second friction elements C and B respectively. In addition, a bypass line 132 is formed on the forward pressure line 118 and a bypass line 134 is formed on the reverse pressure line 120. Check valves 136 and 138 are provided on the bypass lines 132 and 134 respectively, the check valves 136 and 138 opening during the exhaust of hydraulic pressure to enable the quick exhaust of the hydraulic pressure. A safety valve 140 is provided on the reverse pressure line 120. The bypass line 134, the reverse pressure line 120, and the forward pressure line 118 are connected to the safety valve 140.
In the hydraulic control system with the above configuration, gearless shifting is realized in the forward range by variations in the diameter of the primary and secondary pulleys 112 and 108 in a state where hydraulic pressure is being supplied to the first friction element C, while gearless shifting is realized in the reverse range by variations in the diameter of the primary and secondary pulleys 112 and 108 in a state where hydraulic pressure is being supplied to the second friction element B.
However, in such a hydraulic control system, because an accumulator is applied at each of the friction elements C and B, the use of two accumulators utilizes a substantial amount of space, increases the weight of the system, and also acts to increase manufacturing costs.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a hydraulic control system for a continuously variable transmission in which a single accumulator is used to control two friction elements such that the weight and size of the hydraulic control system is reduced, and production costs are minimized.
To achieve the above object, the present invention provides a hydraulic control system for a continuously variable transmission comprising a pressure regulating means including a primary regulator valve for regulating hydraulic pressure supplied from an oil pump, a first solenoid valve, a secondary regulator valve, and a solenoid supply valve; a shift control means including a shift ratio control valve, and a second solenoid valve controlling the shift ratio control valve; a forward/reverse control means including a pressure control valve, a third solenoid valve controlling the pressure control valve, a manual valve, and first and second friction elements acting respectively as forward and reverse operational elements; and a torque converter operation control means including a torque converter feed valve receiving hydraulic pressure from the pressure regulating means, a lock-up clutch control valve, and a fourth solenoid valve controlling the lock-up clutch control valve, wherein a single accumulator is mounted between the pressure control valve and the manual valve of the forward/reverse control means.
According to a feature of the present invention, a bypass line is formed between the pressure control valve and the accumulator, and a check valve is provided on the bypass line, the check valve being able to block the flow of hydraulic pressure through the bypass line.